A structure consisting of several dozen fuel rods that is inserted into the core of a nuclear reactor. Each type of reactor has its own type of fuel assembly. In general, it is a bundle of dozens of fuel rods several meters long. Each rod consists of a cladding tube containing fuel pellets. The fuel assembly often includes control rods or channels for their movement. The arrangement of the fuel rods in the fuel assembly allows for the flow of coolant. Each fuel assembly is inserted at a designated position in the core. At the end of its life in the reactor, the entire assembly is removed and treated as spent fuel.